This invention relates to the manufacture of flat glass wherein the glass is formed into a flat sheet while supported on a pool of molten metal, commonly referred to as the float process. More particularly, this invention relates to a process for sizing the glass by means of pressure while supported on the molten metal so as to minimize distortion in the product glass.
In a float forming process molten glass is delivered onto a pool of molten metal, usually tin or an alloy thereof, and thereafter formed into a continuous ribbon or sheet of glass. Under the competing forces of gravity and surface tension, the molten glass on the molten metal spreads outwardly to an equilibrium thickness of about 6.6 millimeters. In order to produce glass of thicknesses less than the equilibrium thickness the prior art has resorted to various arrangements for stretching the glass ribbon while still in a viscous state on the molten metal. These arrangements usually involve engaging marginal edge portions of the ribbon with mechanical devices, usually toothed rolls. The contact between the glass ribbon and these mechanical devices is believed to create disturbances in the ribbon as well as the molten metal pool which cause optical distortion to be imparted to the glass. It would be highly desirable to eliminate the disturbances caused by attenuating devices and to thereby improve the optical quality of the glass produced.
The use of super-atmospheric gas pressure for attenuating glass has been suggested in the prior art, for example, in U.S. Pat. Nos. 3,241,937 (Michalik et al.); 3,241,938 (Michalik); 3,241,939 (Michalik); 3,248,197 (Michalik et al.); 3,345,149 (Michalik et al.); 3,615,315 (Michalik et al.); 3,749,563 (Stingelin); 3,883,338 (Stingelin); 3,885,944 (Stingelin); 3,432,283 (Galey). In each of these prior art arrangements a higher pressure is maintained over central portions of the glass ribbon than along marginal regions of the ribbon. This entails use of a plenum pressurized with gas overlying the ribbon of the glass and having edges closely spaced above the glass ribbon defining a peripheral slot through which the pressurized gas escapes. Because of the large volume of gas escaping, such arrangements have been less practical than would be desired for widespread commercial application. In one of the above mentioned U.S. patents, No. 3,432,283, there is shown an auxiliary pressure sizing chamber for speeding the spreading of the initially deposited mass of molten glass. However, since the mass of glass is initially very thick, a subsequent pressure sizing chamber is required in order to obtain the desired less than equilibrium thickness in the glass sheet. Instead of enlarging the pressure sizing chamber as in that patent, it would be desirable to size the glass in as short a length as possible in order to minimize the size of the molten metal bath and to minimize the volume of pressurized gas that must be supplied for the sizing process. Because the atmosphere in the forming chamber is a non-oxidizing gas in order to avoid oxidation of the molten metal, minimizing the volume used is an important cost factor. Furthermore, pressure sizing the glass from a relatively thick initial deposit on the molten metal as in the prior art requires pressures within the pressure sizing chambers greater than would be desired. High pressures within the pressure sizing chamber lead to high velocity escape of gases through the peripheral openings between the pressure chamber walls and the glass in the prior art arrangements which, in turn, leads to detrimentally high gas usage. Unduly large volumes of gas throughput can also lead to excessive cooling of the forming chamber unless considerable amounts of energy are employed to preheat the gas.
U.S. Pat. No. 3,841,857 discloses a method for attenuating glass by blasts of gas on both sides of a glass ribbon. Such an approach, however, foregoes the benefits of a molten metal float bath for providing surface smoothness.